This invention relates to a method of development with a liquid developer used in electrophotography and, more particularly, to a method of making a printing plate.
An electrophotographic offset master such as zinc oxide photosensitive paper is converted into a printing plate through the processing steps of negative charging, imagewise exposure, liquid development, and fixing. The liquid developer used in the processing is a positive charge type. The excess liquid developer remained on the developed surface of the master is removed immediately after the development by squeezing through a pair of squeeze rolls. It is naturally desirable that there be plenty of toner particles on the image areas sufficient for receiving a necessary amount of the printing ink and none on the non-image areas. The toner particles adhered to the non-image areas, which cause fogging, are generally originated from the development step. In some plate making units, however, fogging takes place frequently after the development. For instance, in a unit employing a pair of squeeze rolls different from each other in construction material, the rolls acquire static charges due to triboelectricity under conditions of low temperature and especially low humidity which prevail in winter, and there is formed a line of electric force between two rolls which induces the electrophoresis of toner particles to allow the latter to adhere to the roll.
In a particular case of a pair of squeeze rolls comprising a metal roll and an elastic roll such as rubber roll, which are arranged so that the metal roll may be brought into contact with the photosensitive surface of the master, it seems that although both rolls acquire positive charges due to the triboelectricity, yet the positive charge on the elastic roll is larger, resulting in adherence of toner particles having positive charges to the metal roll. The toner particles adhered to the metal roll and gradually accumulated thereon are transferred to the photosensitive surface of the master to cause background staining (fogging).
The toner particles are generally imparted with positive charge by charge controlling agents such as polyvalent metal salts of resin acid or fatty acid. Accordingly, it might be considered possible to overcome the above difficulty by reducing the amount of charge controlling agents, thereby to decrease the positive charge. However, the reduction in the quantity of charge controlling agent is not effective to solve the problem, but rather enhances the fogging or causes an excessive toner adhesion to the image areas which results in additional disadvantage of tailing and image distortion.